1. Field of the Invention
The subject invention relates generally to a device for dispensing small volumes of liquids in the form of droplets and more specifically to a dispensing device which utilizes a source of compressed air to eliminate start-up transience.
2. Background Art
Many analytical techniques require the accurate and precise application or delivery of small volumes of liquid samples. In order to meet these needs, various syringe-based dispensers have been designed, K. R. Millar, F. Cookson & F. M. Gibb, Lab. Pract., 28 (1979) 752; E. H. Pals, D. N. Baxter, E. R. Johnson & S. R. Crouch; Chem., Biomed., & Environ. Instr., 9 (1979) 71; V. Sacchetti, G. Tessari & G. Torsi, Anal. Chem., 48 (1976) 1175. However, these devices are generally limited to delivering volumes of one microliter or larger and are not amenable to rapid, electronic control of the volume dispensed. They also often suffer from irreproducible transfer of the sample to a surface, such as that of an electrothermal atomizer, F. J. M. J. Maessen, F. D. Posma & J. Balke, Anal. Chem., 46 (1974) 1445.
Tiny samples in the form of microdroplets, typically 50-100 micrometers in diameter, were used by several researchers in the study of atomization processes in chemical flames G. M. Hieftje & H. V. Malmstadt, Anal. Chem., 40 (1968) 1860; G. M. Hieftje & H. V. Malmstadt, Anal. Chem. 41 (1969) 1735; B. M. Joshi & R. D. Sacks, Anal. Chem., 51 (1979) 1781, and as a means of sample introduction for quantitative analysis, G. J. Bastiaans & G. M. Hieftje, Anal. Chem., 45 (1973) 1994. Microdroplets have also been employed for titrant delivery in micro-titrations, G. M. Hieftje & B. M. Mandarano, Anal. Chem. 44 (1972) 1616; T. W. Hunter, J. T. Sinnamon & G. M. Hieftje, Anal. Chem., 47 (1975) 497.
The use of a microdroplet generator for sample delivery is attractive primarily because of the wide range of volumes which can be accurately dispensed and the ease with which this volume can be controlled by varying the number of droplets generated. Unfortunately, most devices used to generate microdroplets are not convenient to use and require substantial bulk volumes from which the droplets are extracted. Such devices form droplets by forcing the desired solution through a vibrating capillary or orifice and sonically decomposing the resulting jet into a stream of droplets. This method requires relatively large amounts of sample solution, is prone to failure from capillary clogging, and expels microdroplets with considerable velocity, making them hard to control and encouraging droplet splashing or shattering. In addition, microdroplet generators also suffer from a significant level of hysteresis upon start-up which adversely affects the accuracy of liquid volumes initially produced by the generator. The prior art offers no satisfactory method for dealing with these initial, non-uniform microdroplets.